There are prior art techniques for local modification of magnetic material in the fabrication of magnetic devices. Lawrance, U.S. Pat. No. 3,314,056, "Gapless Magnetic-Head", issued Apr. 11, 1967, discloses a magnetic head having a ring core in which the gap is formed by treatment to reduce the permeability in a restricted core region. Neutron bombardment is the preferred method to significantly diminish the magnetic properties in the gap region. The impact of the neutrons serves to strain the existing crystal structure of the localized region so that the magnetic properties of the core material in the treated region are affected.
Boll, U.S. Pat. No. 4,265,684, "Magnetic Core Comprised of Low-retentivity Amorphous Alloy", issued May 5, 1981, discloses a magnetic head having a ring core made from an amorphous alloy. An effective gap is produced in the core by converting the material in a local region to the crystalline state by localized heating to a temperature above the crystallization temperature of about 400.degree. C.
Otomo et al., U.S. Pat. No. 4,772,976, "Process for Preparing Magnetic Layer and Magnetic Head Prepared Using the Same", issued Sep. 20, 1988, describe a thin film magnetic head in which ion implantation is used to locally vary the magnetic characteristics of the magnetic material. The ions are then thermally diffused into the material by heat treatment to attain uniform magnetic characteristics.
L. Schultz et al., "Permanent Local Modification of the Magnetic Bubble Properties of Epitaxial Garnet Films By Laser Annealing," Journal of Applied Physics, Vol. 50, No. 9, September 1979, describe laser annealing of magnetic materials. These studies were aimed toward using a laser to locally increase the magnetization of garnet films for channeling of magnetic bubbles. The changes in magnetic properties that were observed were attributed to a crystallographic site redistribution of the Ga and Fe atoms in annealed regions which support mobile bubbles.
None of the cited references shows a magnetic structure comprising a layer of magnetic material with an overlayer and an underlayer and heat treatment to produce interaction between the materials of the overlayer and the underlayer with the layer of magnetic material to produce selected magnetic and electrical characteristics in localized areas of the layer of magnetic material. However, there is prior art which deals with the local modification of a magnetic structure with an overlayer.
Bajorek et al., U.S. Pat. No. 3,840,898, "Self-biased Magnetoresistive Sensor", issued Oct. 8, 1974, describe a magnetoresistive sensor in which a protective coating of photoresist is deposited over a preselected portion of an NiFe film, and the structure is then annealed at elevated temperatures in an oxygen-rich atmosphere. The resultant oxidation of the unprotected part of the NiFe film results in depletion of Fe from the bulk of the unprotected part of the film so as to effectively change its composition, and also change its composition-dependent magnetic characteristics.
Bajorek et al., U.S. Pat. No. 3,887,944, "Method for Eliminating Part of Magnetic Crosstalk in Magnetoresistive Sensors", issued Jun. 3, 1975, disclose an integrated array of side-by-side magnetoresistive (MR) reading heads. To eliminate crosstalk between adjacent MR read heads, a region of high-coercivity material is formed between adjacent MR read heads. The region of high-coercivity material can be produced by exchange coupling through alloying or other chemical reaction, or by chemical treatment, to roughen the MR stripe prior to depositing a conductor into the roughened area.
However, Bajorek et al., in the above two patents, do not teach a purposeful selection of an underlayer as well as an overlayer in producing modification of their magnetic structure. Also, they do not consider the possibility of producing bias fields, the direction of such bias fields, if they do exist, or the subsequent domain states of the MR sensor.